diff --git a/eive_tmtc/tmtc/acs/star_tracker.py b/eive_tmtc/tmtc/acs/star_tracker.py
index 28e9936..1923613 100644
--- a/eive_tmtc/tmtc/acs/star_tracker.py
+++ b/eive_tmtc/tmtc/acs/star_tracker.py
@@ -95,7 +95,10 @@ class StarTrackerActionId(enum.IntEnum):
     LOGLEVEL = 81
     LOG_SUBSCRIPTION = 82
     DEBUG_CAMERA = 83
+    # Legacy variable.
     FIRMWARE_UPDATE = 84
+    FIRMWARE_UPDATE_MAIN = 84
+    FIRMWARE_UPDATE_BACKUP = 101
     SET_TIME_FROM_SYS_TIME = 87
     ADD_SECONDARY_TM_TO_NORMAL_MODE = 95
     RESET_SECONDARY_TM_SET = 96
@@ -119,9 +122,10 @@ class OpCode:
     UPLOAD_IMAGE = "upload_image"
     DOWNLOAD_IMAGE = "download_image"
     SET_IMG_PROCESSOR_MODE = "set_img_proc_mode"
-    FW_UPDATE = "fw_update"
+    FW_UPDATE_MAIN = "fw_update_main"
     SET_TIME_FROM_SYS_TIME = "set_time"
     RELOAD_JSON_CFG_FILE = "reload_json_cfg"
+    FW_UPDATE_BACKUP = "fw_update_backup"
 
 
 class Info:
@@ -135,7 +139,8 @@ class Info:
     DOWNLOAD_IMAGE = "Download Optical Image"
     TAKE_IMAGE = "Take Image"
     SET_IMG_PROCESSOR_MODE = "Set Image Processor Mode"
-    FW_UPDATE = "Firmware Update"
+    FW_UPDATE_MAIN = "Update Main Firmware Slot"
+    FW_UPDATE_BACKUP = "Update Backup Firmware Slot"
     SET_TIME_FROM_SYS_TIME = "Set time from system time"
     RELOAD_JSON_CFG_FILE = "Reload JSON configuration file. Reboot still required."
 
@@ -642,12 +647,21 @@ def pack_star_tracker_commands(  # noqa C901
             + bytearray(json_file, "utf-8")
         )
         q.add_pus_tc(PusTelecommand(service=8, subservice=128, app_data=data))
-    if cmd_str == OpCode.FW_UPDATE:
-        q.add_log_cmd(Info.FW_UPDATE)
+    if cmd_str == OpCode.FW_UPDATE_MAIN:
+        q.add_log_cmd(Info.FW_UPDATE_MAIN)
         firmware = get_firmware()
         data = (
             obyt
-            + struct.pack("!I", StarTrackerActionId.FIRMWARE_UPDATE)
+            + struct.pack("!I", StarTrackerActionId.FIRMWARE_UPDATE_MAIN)
+            + firmware.encode()
+        )
+        q.add_pus_tc(PusTelecommand(service=8, subservice=128, app_data=data))
+    if cmd_str == OpCode.FW_UPDATE_BACKUP:
+        q.add_log_cmd(Info.FW_UPDATE_BACKUP)
+        firmware = get_firmware()
+        data = (
+            obyt
+            + struct.pack("!I", StarTrackerActionId.FIRMWARE_UPDATE_BACKUP)
             + firmware.encode()
         )
         q.add_pus_tc(PusTelecommand(service=8, subservice=128, app_data=data))
@@ -1185,7 +1199,7 @@ def create_str_node() -> CmdTreeNode:
     node.add_child(
         CmdTreeNode(OpCode.RESET_SECONDARY_TM_SET, Info.RESET_SECONDARY_TM_SET)
     )
-    node.add_child(CmdTreeNode(OpCode.FW_UPDATE, Info.FW_UPDATE))
+    node.add_child(CmdTreeNode(OpCode.FW_UPDATE_MAIN, Info.FW_UPDATE_MAIN))
     node.add_child(
         CmdTreeNode(OpCode.SET_TIME_FROM_SYS_TIME, Info.SET_TIME_FROM_SYS_TIME)
     )
@@ -1212,7 +1226,7 @@ def add_str_cmds(defs: TmtcDefinitionWrapper):
     )
     oce.add(OpCode.READ_SECONDARY_TM_SET, Info.READ_SECONDARY_TM_SET)
     oce.add(OpCode.RESET_SECONDARY_TM_SET, Info.RESET_SECONDARY_TM_SET)
-    oce.add(OpCode.FW_UPDATE, Info.FW_UPDATE)
+    oce.add(OpCode.FW_UPDATE_MAIN, Info.FW_UPDATE_MAIN)
     oce.add(OpCode.SET_TIME_FROM_SYS_TIME, Info.SET_TIME_FROM_SYS_TIME)
     oce.add(OpCode.RELOAD_JSON_CFG_FILE, Info.RELOAD_JSON_CFG_FILE)
     defs.add_service(CustomServiceList.STAR_TRACKER.value, "Star Tracker", oce)